High efficiency carbon nanotubes-based single-atom catalysts for nitrogen reduction

Carbon-based single-atom catalysts (SACs) for electrochemical nitrogen reduction reaction (NRR) have received increasing attention due to their sustainable, efficient, and green advantages. However, at present, the research on carbon nanotubes (CNTs)-based NRR catalysts is very limited. In this paper, using FeN3@(n, 0) CNTs (n = 3 ~ 10) as the representative catalysts, we demonstrate that the CNT curvatures will affect the spin polarization of the catalytic active centers, the activation of the adsorbed N2 molecules and the Gibbs free energy barriers for the formation of the critical intermediates in the NRR processes, thus changing the catalytic performance of CNT-based catalysts. Zigzag (8, 0) CNT was taken as the optimal substrate, and twenty transition metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Tc, Ru, Rh, Pd, W, Re, Ir, and Pt) were embedded into (8, 0) CNT via N3 group to construct the NRR catalysts. Their electrocatalytic performance for NRR were examined using DFT calculations, and TcN3@(8, 0) CNT was screened out as the best candidate with a low onset potential of − 0.53 V via the distal mechanism, which is superior to the molecules- or graphene-support Tc catalysts. Further electronic properties analysis shows that the high NRR performance of TcN3@(8, 0) CNT originates from the strong d-2π* interaction between the N2 molecule and Tc atom. TcN3@(8, 0) CNT also exhibits higher selectivity for NRR than the competing hydrogen evolution reaction (HER) process. The present work not only provides a promising catalyst for NRR, but also open up opportunities for further exploring of low-dimensional carbon-based high efficiency electrochemical NRR catalysts.


Scientific Reports
| (2023) 13:9926 | https://doi.org/10.1038/s41598-023-36945-0 www.nature.com/scientificreports/ be regarded as a curved graphene surface, which provides a potential means to use this feature to tune the catalytic performance. However, there are still relatively few researches on CNT-based SACs in the field of NRR 17,34 . Therefore, it is critical to investigate the structures, catalysis performance and reaction mechanisms of SACs based on CNTs to advance the development of highly efficient electrocatalysts for NRR. In this work, using the density functional method, the effect of CNT curvatures on the catalytic performance of CNT based catalysts for NRR were investigated firstly. According to the criteria proposed for the screening of eligible electrocatalysts for NRR, zigzag (8,0) CNT was selected as the optimal substrate to construct CNT based NRR catalysts. Through screening twenty transition metal atoms, Tc atom were found to exhibit the best N 2 to NH 3 conversion capabilities via the distal pathway with the extremely low limiting potential (− 0.53 V). Further calculations were performed to investigate the electronic properties to explain the high NRR performance of TcN 3 @(8, 0) CNT, and to evaluate the selectivity between NRR and hydrogen evolution reaction (HER).

Computational details
All calculations are based on density functional theory as implemented in the Vienna ab initio simulation package (VASP) 35 . Our exchange-related functional adopts the revised Perdew-Burke-Ernzerhof (rPBE) 36 under the generalized gradient approximation (GGA) method 37 . The projector augmented wave (PAW) method 38 is used to describe the ion-electron interaction, and the PAW cutoff is set to 550 eV. The van der Waals interaction is calculated using the DFT-D3(IVDW = 11) method 39 . 1 × 1 × 3 supercells of carbon nanotubes are used to construct the structure models. In order to eliminate the interactions between two periodic repeating structures, the lattice parameters in the vacuum directions are set as 25.0 Å. The Brillouin zone is sampled using the Monkhorst-Pack k-point mesh 40 of 1 × 1 × 2. All the structures are fully optimized until the energy convergence standard of 10 -5 eV and the force convergence standard of − 0.01 eV/Å are reached. To further investigate the structural stability of the most likely carbon nanotube-supported monatomic catalysts, Ab initio molecular dynamics (AIMD) simulations in the canonical ensemble (NVT) with the Nose´-Hoover thermostat 41 were performed at 500 K for 5 ps with a time-step of 1.0 fs.
The Gibbs free energy change (ΔG) in each elementary step is calculated based on the computational hydrogen electrode (CHE) model proposed by Norskov et al. 42 . The free energy of a proton-electron pair (H + + e − ) is equivalent to 1/2 H 2 (g) under standard reaction conditions (pH = 0, 298.15 K, 101.325 kPa) at an external potential of 0 V. The free energy of the H* (ΔG H ) is calculated to be − 0.62 eV in this work. The following equation is used for the calculations 43 : Among them, ΔE is the electron energy difference between two intermediates; ΔZPE is the change in zero point energy; T is the temperature (298.15 K); ΔS is the change in entropy calculated by frequency; ΔG U = − eU, it represents the contribution of the electrode potential U to the free energy, where e is the number of transferred electrons, and U is the applied electrode potential; ΔG pH = k B T × ln10 × pH, it represents the free energy correction of pH, where k B is the Boltzmann constant, and pH value is set to be zero.
In addition, the adsorption energy is defined as: where E ads-sub represents the total energy of the system after adsorption, and E ads is the total energy of the adsorbent, E sub represents the total energy of the substrate.

Results and discussion
Effect of CNT curvature on the catalytic performance of CNT based catalysts. Initially, eight zigzag (n, 0) CNTs (n = 3-10), were considered as substrates to anchor transition metal (TM) atoms to construct single atom catalysts. As shown in Fig. 1a, on the surface of a CNT, one carbon atom is deleted to form a singlevacancy defect, and then three carbon atoms possessing dangling bonds are substitutionally doped with nitrogen atoms to form a N 3 group. The TM atoms are adsorbed at the center of the N 3 groups. Previous studies have shown that FeN 3 -embedded graphene exhibits excellent catalytic performance for the N 2 -to-NH 3 conversion 13 . Therefore, Fe was employed as the representative TM atom to study the effect of CNT curvature on the NRR catalytic performance of TMN 3 @(n, 0) CNTs. The spin magnetic moment of the Fe center in FeN 3 @graphene is proved to be critical for the activation of the inert N 2 molecule by the catalysts 13 . Therefore, the spin-resolved density of FeN 3 @(n, 0) CNTs were investigated firstly, and the results (Fig. 1b-i) demonstrate that all the FeN 3 centers are highly spin-polarized. The Fe atoms protrude outside the CNT surfaces, and the charge clouds is distributed near the Fe atoms, indicating that the Fe atoms contribute most of the spin moments. With the decrease of the CNT curvatures from (3, 0) CNT to (10, 0) CNT, the spin magnetic moment decreases from 3.43 to 3.02 µ B , approaching that of FeN 3 @graphene. These results suggest that the curvatures of the substrates anchoring Fe atoms can affect the spin polarization of Fe atoms, and the larger the curvature, the greater the degree of the spin polarization. Figure 2a demonstrates the variation of the N ≡ N bond lengths of N 2 molecules adsorbed on FeN 3 @(n, 0) CNTs. There is a good linear relationship between the change of the N ≡ N bond lengths and the CNT curvatures, indicating that the CNT curvature plays an important role in weakening the N ≡ N bond. After adsorption, the N ≡ N bond is stretched from 1.10 Å in free N 2 molecule to 1.129-1.141 Å, indicating that N 2 is effectively activated by FeN 3 @(n, 0) CNTs. With the decrease of the CNT curvatures from (3, 0) CNT to (6, 0) CNT, the N ≡ N bond length increases from 1.129 to 1.141 Å. While the elongation of the N ≡ N bond by FeN 3 @(n, 0) (2) E ads = E ads−sub − E ads − E sub , www.nature.com/scientificreports/ CNTs (n = 7-10) are very similar, ranging from 1.138 to 1.141 Å. Therefore, reducing the curvature of CNTs can improve the activation of N 2 molecules by FeN 3 to a certain extent. NRR process is complicated and can occur through distal, alternating, enzymatic and other mechanisms. During the NRR process, the effective adsorption of N 2 on active sites of the catalyst and its activation are prerequisites. The chemisorption of N 2 will make sufficient activation of the inert N≡N triple bond. According to previous studies, the formation of *N 2 H and *NH 3 usually have relatively high free energies, thus making them the potential determining steps. Therefore, the following criteria have been proposed for the screening of eligible electrocatalysts for NRR 12 : (1) The adsorption energies of N 2 should be lower than − 0.50 eV, corresponding to the chemisorption of N 2 molecule, so as to ensure the effective activation of the inert N-N triple bonds; (2) The Gibbs free energy changes in the conversion processes of *N 2 to *N 2 H ( G N 2 −N 2 H ) and *NH 2 to *NH 3 ( G NH 2 −NH 3 ) should be lower than 0.55 eV to achieve an onset potential comparable to or lower than the predicted onset potential of the most efficient catalysts made of pure transition metals 44 . Therefore, in the following study, these three key steps in the N 2 reduction reactions catalyzed by FeN 3 @(n, 0) CNTs, rather than all the reaction steps, were calculated to investigate the effect of the CNT curvatures on the NRR catalytic performance of FeN 3 @(n, 0) CNTs.
In Fig. 2b, the obtained adsorption energies of N 2 on FeN 3 @(n, 0) CNTs, the Gibbs free energy changes G N 2 −N 2 H and G NH 2 −NH 3 are shown. The N 2 molecule can be adsorbed stably on all the FeN 3 @(n, 0) CNTs structures, but only the adsorption energy on the smallest (3, 0) CNT is higher than − 0.50 eV. With the decrease of the curvatures from (3, 0) to (10, 0) CNT, the adsorption of N 2 becomes more stable. From (8, 0) to (10, 0) CNT, the adsorption energies vary in a very small range. This is because as the CNT diameter increases, the curvature of CNTs decreases, and their surfaces gradually become a plane. Therefore, the adsorption energy will gradually approach a limit, that is, the adsorption energy of N 2 on FeN 3 @graphene. These results demonstrate that, except for CNTs with very small diameters, most other CNTs-based catalysts can adsorb N 2 stably to ensure the effective activation of the inert N-N triple bonds.  www.nature.com/scientificreports/ For the formation of *N 2 H, the effect of CNT curvatures is very small. G N 2 −N 2 H varies in a very small range of 0.78-0.92 eV, indicating that all the FeN 3 @(n, 0) CNTs are inefficient for the catalysis of NRR. On the other hand, the CNT curvatures have a significant effect on the final protonation step of forming the *NH 3 intermediate. From (3, 0) to (5, 0) CNT, G NH 2 −NH 3 increases sharply from − 0.90 to − 0.40 eV. While, from (5, 0) to (10, 0) CNT, G NH 2 −NH 3 increases much slowly from − 0.40 to − 0.16 eV. This shows that the decrease of the CNT curvatures will increase the Gibbs free energy barrier of the formation of the *NH 3 intermediate, thus reducing the performance of the CNTs-based NRR catalyst.
After carefully examining the influence of CNT curvatures on the spin polarization of the Fe centers, the activation of the adsorbed N 2 molecules, and three critical steps, we have selected (8, 0) CNT as the optimal substrate to anchor TM atoms to construct the computational models. Figure 1a shows the optimized structure of FeN 3 @(8, 0) CNT. The bond length between the Fe atom and the upper N atom is 1.84 Å, and the bond lengths of the other two Fe-N bonds are 1.95 Å. Owing to the relatively compact N 3 pores, the adsorption site of Fe atom is slightly elevated above the plane composed of N atoms.
Screening of TM atoms. Next, 20 TM atoms (TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Tc, Ru, Rh, Pd, W, Re, Ir and Pt) anchored at the N 3 center of (8, 0) CNT were screened to obtain an electrochemical NRR catalyst with excellent performance. The relative stability of TMN 3 @(8, 0) CNTs were evaluated by calculating the binding energies (E b ) using the following equation: where E TM@CNT , E CNT and E TM represent the energies of TMN 3 @(8, 0) CNTs, the (8, 0) CNT, and the isolated single TM atoms, respectively. As shown in Fig. 3a and Supplementary Table S1, all the calculated binding energies are negative, indicating that all the 20 TM atoms can be stably adsorbed by (8, 0) CNT. The binding energy of Zn atom is the weakest, while that of Sc atom is the strongest.
Following the above-mentioned criteria, the catalytic performance of TMN 3 @(8, 0) CNTs was then investigated. The adsorption energies of N 2 on the TM atoms in both end-on and side-on fashions are shown in Fig. 3b and Supplementary Table S1. The results show that the end-on configuration is energetically more favorable than the side-on configuration for all TM atoms except for W atom. The capture of N 2 via side-on configuration can www.nature.com/scientificreports/ not meet criterion 1 except for Mo, W, Re, Ir, and Pt. While, in the end-on configurations, only Sc, Mn and Zn are eliminated because of the higher adsorption energies. Therefore, the end-on configuration for N 2 adsorption is employed to investigate the Gibbs free energy changes of the formation of *N 2 H and *NH 3 , and the results are shown in Fig. 3c and Supplementary Table S1. For the formation of *N 2 H, most of the TM atoms are ruled out as NRR catalysts because of the large Gibbs free energy change except for Nb, Mo, Tc, W, and Re. Further considering the formation of *NH 3 , Nb, Mo, W, and Re are ruled out. Eventually, TcN 3 @(8, 0) CNT is the only high-performance NRR candidate catalyst that meets all the above criteria. Moreover, ab initio molecular dynamics (AIMD) simulations using canonical (NVT) ensemble is used to investigate the thermal stability of TcN 3 @(8, 0) CNT structure. After heating at the temperature of 500 K for 5 ps with a time step of 1 fs, we found that the structural reconstruction did not take place, implying that the TcN 3 @ (8, 0) CNT structure can withstand temperature as high as 500 K. The variations of energy with respected to the time for AIMD simulations, and the snapshots of initial and final atomic configurations during the AIMD simulations are shown in Supplementary Fig. S1. 3 @(8, 0) CNT. The full NRR processes catalyzed by TcN 3 @(8, 0) CNT is further investigated via three possible pathways, including distal, alternating and enzymatic mechanisms. The schematic diagrams and the optimized structures of the intermediates in the three mechanisms are depicted in Fig. 4. The full reaction processes can be divided into seven steps, including the first step of N 2 adsorption and six consecutive protonation steps. The distal and alternating mechanisms starts with the N 2 end-on adsorption. In the distal mechanism, the distal N atom in the adsorbed *N 2 will be fully hydrogenated via accepting three proton-electron pairs until the first NH 3 molecule is released, then the proximal N atom continues to accept three proton-electron pairs to form the second NH 3 molecule to complete the whole catalytic process. In the alternating mechanism, the remote and the proximal N atoms of the adsorbed *N 2 alternately accept the proton-electron pairs to form NH 3 molecules. The enzymatic mechanism starts with the N 2 side-on configuration, and the six proton-electron pairs will be attached alternately to the two N atoms as in the alternating mechanism. Figure 5a-c are the calculated Gibbs free energy diagrams for the three possible NRR mechanisms. And the corresponding Gibbs free energy barriers are list in Supplementary Table S2. Our results show that the ΔG values of N 2 adsorption via end-on and side-on fashions are − 0.01 eV and 0.59 eV, respectively. Compared with the side-on adsorption configuration, the end-on adsorption is more advantageous in terms of energy. Due to the extremely stable N-N triple bond in N 2 molecule, breaking it to achieve protonation is often accompanied by a certain energy barrier. From the end-on configuration, the first protonation step requires an energy input of 0.53 eV. Then, the following protonation can take place through two paths, namely, the distal and alternating pathways. In the distal pathway, the energy consumption of the second protonation step (formation of the *N 2 H 2 intermediate) is 0.03 eV. In the subsequent steps, the release of the first NH 3   When the NRR occurs via the alternating pathway (Fig. 5b), the formation of the *N 2 H 2 and *NH 2 NH 2 intermediates are endothermal reactions and they require energy inputs of 0.80 and 0.26 eV, respectively. The second protonation step (*N 2 H → *N 2 H 2 ) becomes the PDS for the alternating mechanism. In the enzymatic path, the maximum energy barrier of the six protonation steps is only 0.29 eV, and the N 2 adsorption (0.59 eV) is the PDS. Here, we use the onset potential (U, its value is defined as: U = − ΔG max /e, where ΔG max is the free energy variation of the PDS in each pathway) as a measure of the NRR performance. The onset potentials of the three possible mechanisms are − 0.53 V, − 0.80 V and − 0.59 V, respectively. Therefore, the distal mechanism is the most energetically favorable pathway for N 2 reduction to ammonia catalyzed by TcN 3 @(8, 0) CNT. Previously, Tc atom has been reported to be the active center of catalyst for NRR. For example, tetracyanoquinodimethane molecules-supported Tc atom (Tc-rTCNQ) can catalyze NRR with a limiting potential of − 0.65 V 45 , while the limiting potential of Tc@N 6 -Graphene is − 0.56 V 46 . It can be observed that the utilization of suitable CNT as a substrate to adsorb Tc atom can effectively improve its catalytic performance. Figure 5d shows the variation of N-N and Tc-N bond lengths in each step of the distal pathway. The N-N bond is firstly stretched from 1.10 Å in free N 2 molecule to 1.15 Å in the *N 2 intermediate, indicating that N 2 is effectively activated by the adsorption on TcN 3 @(8, 0) CNT. Then the N-N bond is gradually elongated by protonation and finally breaks at the third protonation step. The Tc-N bond length is strongly correlated with the release of the first NH 3 molecule. Before the formation of the *N intermediate, the Tc-N bond length decrease stepwise. And after that, the Tc-N bond length increases gradually until the release the second NH 3 molecule.

NRR catalyzed by TcN
We further explore the charge density difference (Fig. 6a) and the variation of the projected density of states (PDOS) of N 2 before and after adsorption in the distal mechanism (Fig. 6b-d). As shown in Fig. 6a, the results of charge density difference indicate that N 2 molecule can interact with the catalyst and the charge transfer is obvious when adsorption occurs. The charges mainly accumulate on the N atoms, while the charges between the N-N is dissipated, indicating that the N 2 molecule is activated and the strength of N-N bond is weakened. The charge accumulation and depletion between TcN 3 @(8, 0) CNT and N 2 can be explained via the donation-back donation mechanism, in which the unoccupied d orbitals of Tc can accept electrons from the occupied orbitals of N 2 , simultaneously the d orbitals electrons of Tc can be transferred to the antibonding orbitals of N 2 . This can be confirmed by the PDOS of N 2 molecule before and after adsorption on TcN 3 @(8, 0) CNT as shown in Fig. 6b-d. The degenerate 2π orbital and 2π* orbital in free N 2 molecule split into individual occupied orbitals after the adsorption of N 2 on the TcN 3 @(8, 0) CNT surface. Some electrons are transferred from the 3σ and spilt 2π orbitals to the unoccupied d orbitals of Tc atom, which can effectively enhance the adsorption of N 2 molecule. Meanwhile, the 2π* antibonding orbitals split into two parts, i.e., occupied and unoccupied orbitals, in which the electrons of the occupied orbitals originate from the back donation of the occupied d orbitals of Tc. The strong www.nature.com/scientificreports/ d-2π* interaction between N 2 molecule and Tc atom is the key to promote the activation of the N 2 molecule. Therefore, the adsorbed N 2 on TcN 3 @(8, 0) CNT surface can be activated efficiently, which also explains why TcN 3 @(8, 0) CNT can efficiently catalyze the N 2 reduction reaction via distal mechanism.
Charge variation in the distal pathway and HER competition. In order to further understand the superior NRR catalytic performance of TcN 3 @(8, 0) CNT, we calculated the Bader charge variation of the reaction intermediates in the favorable distal mechanism and the results are shown in Fig. 7 and Supplementary  Table S3. According to previous studies [47][48][49] , we divided each intermediate in this process into three parts: moiety 1 (carbon nanotube), moiety 2 (TcN 3 ), and moiety 3 (the adsorbed N x H y species). The charge variation refers to the charge difference of each moiety between the current step and the previous step. The charge variation at the first step demonstrates that the N 2 molecule obtains 0.40e from CNT and TcN 3 moieties during the end-on fashion adsorption. Both CNT and TcN 3 play an important role in the activation process of the adsorbed N 2 molecule. TcN 3 donates electrons to both CNT and moiety 3 at the second step. At the third and fourth steps, the charge variation of TcN 3 is about zero, and the charge variation of moiety 1 and moiety 3 is complementary to each other. While TcN 3 and moiety 3 get almost the same amount of electrons from CNT at the fifth and the  www.nature.com/scientificreports/ sixth step. At the final step, the second NH 3 molecule is formed and moiety 3 returns the excess electrons to the catalyst itself. Based on the above analysis, we found that CNT acts as an electron reservoir in the whole catalytic process, while the active center TcN 3 acts sometimes as a bridge to transport electrons. Both these two parts make an important contribution to the high NRR catalytic performance of TcN 3 @(8, 0) CNT. In addition to the structural stability and high catalytic activity, an idea NRR catalyst should be also able to suppress the HER, a key side reaction in NRR, to achieve the high Faraday efficiency (FE). Therefore, we evaluated the catalytic selectivity of TcN 3 @(8, 0) CNT via two measures. On the one hand, the adsorption energies of N 2 and proton on the catalyst are calculated to be − 0.94 eV and − 0.81 eV, respectively. The adsorption energy results indicate that the adsorption of N 2 molecule is more stable than that of proton, which prevents the adsorption of a large number of protons on the catalyst surface, thus hindering the HER process. On the other hand, the measure of the difference of the limiting potentials between NRR and HER is calculated as: ΔU = U PDS (NRR)-U PDS (HER), where U PDS (NRR) and U PDS (HER) respectively represent the limiting potentials of NRR and HER 16,48 . A positive value represents that HER is suppressed to enhance the selectivity of NRR 50 . Our calculation results show that the limit potential of HER is − 0.62 V. Hence, ΔU is 0.09 V (As shown in Supplementary Fig. S2), which demonstrates its high selectivity to NRR.

Conclusion
In summary, first-principles calculations were performed to explore high efficiency NRR single-atom catalysts based on CNTs. At first, the effect of CNT diameters on the NRR catalytic performance of CNTs based catalysts were investigated. The results showed that CNT curvatures have a significant effect on the spin polarization of the catalytic active centers, the activation of the adsorbed N 2 molecules and the Gibbs free energy barriers for the formation of the *NH 3 intermediate, but have little effect on the formation of *N 2 H intermediate. With the decrease of the CNT curvatures, the adsorption of N 2 will become more effective, but the onset potential of the formation of the *NH 3 intermediate will also increase, thus reducing the performance of the CNTs-based NRR catalyst. Therefore, zigzag (8, 0) CNT was selected as the substrate to anchor twenty TM atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Tc, Ru, Rh, Pd, W, Re, Ir, and Pt) and their catalytic ability were investigated systematically according to the criteria for the screening of eligible electrocatalysts for NRR. TcN 3 @(8, 0) CNT is the only possible candidate catalyst for high performance NRR after screening, and our calculations illustrate that NRR prefers the distal pathway with a low limiting potential of − 0.53 V. The strong d-2π* interaction between the active center and N 2 molecule is the key to facilitate the N 2 molecule activation. Furthermore, TcN 3 @(8, 0) CNT exhibits higher selectivity for NRR than the competing HER process. Considering that Tc is a radioactive material, in the future, a pseudo-Tc material 51 (such as Mo-Ru alloy) may be designed based on the concept of density of states engineering to effectively prevent any potential risks associated with radioactivity while retaining the desirable catalytic properties. We expect that our results would inspire more research on low-dimensional carbonaceous materials in the field of NRR electrocatalysis.

Data availability
Data are available from the corresponding author on reasonable request.